Cubic fluorite structure

This compound has the cubic fluorite structure with one octahedral interstice per Ce atom. Therefore, a =1, and S = 2 for CeH2. We can therefore write ... 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

The parent structure of the anion-deficient fluorite structure phases is the cubic fluorite structure (Fig. 4.7). As in the case of the anion-excess fluorite-related phases, diffraction patterns from typical samples reveals that the defect structure is complex, and the true defect structure is still far from resolved for even the most studied materials. For example, in one of the best known of these, yttria-stabilized zirconia, early studies were interpreted as suggesting that the anions around vacancies were displaced along < 111 > to form local clusters, rather as in the Willis 2 2 2 cluster described in the previous section, Recently, the structure has been described in terms of anion modulation (Section 4.10). In addition, simulations indicate that oxygen vacancies prefer to be located as second nearest neighbors to Y3+ dopant ions, to form triangular clusters (Fig. 4.11). Note that these suggestions are not... 

CeOj and ThOj have the cubic fluorite structure. Fig. 2.15, and can be doped with large amounts of, for example, Ca, La or Gd to give extensive ranges of cubic solid solutions. ZrOj is cubic only above 2400°C, however, and requires 8% of dopant to stabilise the cubic form to room temperature (as in YSZ, yttria-stabilised zirconia). 

Stabilized zirconia refers to a solid solution of zirconium oxide with one or more of a number of stabilizing oxides (CaO, MgO, 20, or others) to form a cubic fluorite structure. This... 

Cubic fluorite-structure (Fm3m) zirconia-based solid solution, (Zr,ACT,REE)02 x, exhibi ts significant compositional flexibility to incorporate high concentrations of Pu, neutron absorbers, and impurities contained in Pu-bearing wastes (Gong et al. 1999). The phase has excellent radiation stability. No amorphization was observed under ion irradiation at room temperature to a dose corresponding to 200 dpa, and at 20 K to a dose of 25 dpa. Irradiation with I+ and Sr+ up to 300 dpa produced defect clusters in Y-stabilized zirconia, but did not cause amorphization. Amorphization... 

In the arsenide and antimonide derivatives Na2AuAs or Na2AuSb there are zigzag (AuX)2 -chains, with structures like the gold(I) halides (4), while AuSb2 has metallic properties and has the cubic fluorite structure.325-327... 

It was rather surprising when Hund and Durrwachter [312] found that La20s is miscible with TI1O2 to a great extent (52 mole per cent) whilst still preserving the cubic fluorite structure. The lattice constant of the mixed oxide has an a value 5.645 A compared to 5.592 A for TI1O2. The lattice constants of some orthorhombic perovskite and cubic garnet-type europium compounds are listed in Table 22. 

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

The size and shape of ceria NCs are proven fo appreciably change the chemical and physical properties hence, their control in synthesis is one chief objective for study, and various nanoparticles, nanocubes, nanooc-tahedra, nanowires, and nanotubes have been obtained for this purpose. Owing to the cubic fluorite structure, ceria tends to form isometric particles, which present sphere-like morphology and are usually intermediates between the shape of cubes and octahedra. The major exposed crystal surfaces for ceria NCs are low index ones, that is, 100, llOj, and 111, with considerable surface relaxation and reconstructions. Figure 1 shows some typical morphologies of ceria NCs. 

The thorium oxide system is dominated by Th02. The dioxide can be synthesized by burning a number of thorium compounds, including hydroxides, oxalates, carbonates, and so on. The Th02 crystalhzes in the cubic fluorite structure. Th02 is very heat resistant as are all of the actinide oxides and melts at 3390 °C, which is the highest for any known metal oxide. 

Protactinium oxides can be stabilized in the tetravalent and pentavalent state. The most stable oxide phase obtained by the burning of metal or protactinium compoimds is the white pentoxide, Pa20s. The structme of the pentoxide is related to fluorite and has cubic symmetry. Pa02 is a black solid that crystallizes in the cubic fluorite structure. 

Raman spectroscopy [24,25] Six Raman-active modes of Aig + 3 Eg + 2 Big symmetry are observed for tetragonal Z1O2 (space group P42/nmc), while for the cubic fluorite structure (space group Fm3m) only one p2g mode centred at around 490 cm is observed for c-Zr02 [22,24,25,32]. An example of the variation of the Raman patterns with composition is reported in Fig.6.4. 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

The oxides UO2, (U, Pu)02, and Th02, which have face-centered-cubic fluorite structures and are completely miscible in solid solution, are the most extensively used... 

Whereas CoSij and NiSij crystallize with the cubic fluorite structure, this structure is distorted in -FeSij to give Fe two Fe neighbours, the metal atoms being arranged in squares of side 2-97 A (compare 2 52 A in the 12-coordinated metal). The metal atom also has 8 Si neighbours at the vertices of a deformed cube. This distortion of the fluorite structure represents a partial transition towards the CuAlj structure (p. 1046), in which the coordination group of Cu is a square antiprism and there are chains of bonded Cu atoms. (AC 1971 B27 1209). 

Pure titania has the rutile structure and therefore has limited solubility in YSZ. The observed linear decrease in lattice parameter with increasing titania concentration in these solid solutions suggests that titanium cations enter the lattice substitutionally for zirconium. Concordant with the data from XRD measurements [29,30,123] the cubic fluorite structure is retained upon addition of 12-20 mol% titania, above which a second phase appears, claimed to be ZrTi04 [123]. The spread in data of the solubility limit produced by different authors may be due to slight differences in, e.g., yttria concentration, sample processing, sintering temperature and impurity content in the cited studies. Microstructural investigations based on SEM and TEM indicated that precipitates of the second phase actually may appear already at lower titania contents [123,125]. 

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